Allergic or hypersensitivity responses to IGIV commonly occur. Most of these reactions consist of fever, chills, dyspnea, hives or angioedema. Rarer, but more dangerous anaphylactic reactions can also occur especially in patients with selective IgA deficiency. These immediate allergic reactions are typically mediated by the recipient's IgE antibodies. Thus allergic reactions to biologics, such as IGIV constitute an important problem when the patient must receive repeated treatments. We have developed a mouse model to determine whether IgE responses to a complex protein can be prevented or down-regulated. Immunoglobulin isotype switching to IgE in human and murine B cells is induced by antigen- specific Th2 cells which secrete IL-4. In contrast, Th1 cells, which secrete IL-2 and IFN-g promote IgG2a antibodies in mice. We have developed a mouse model to investigate whether stimulation of Th1 cells can limit allergic reactions by down-regulating Th1 responses. Heat-killed Brucella Abortus (BA) was used as a Th1-promoting adjuvant in an attempt to suppress IgE responses generated by immunization with an allergenic mixture of ovalbumin + alum (0/A). We found that i) BA prevented IgE responses and markedly enhanced IgG2a when given with 0/A, ii) IgG2a enhancement was IL-12 dependent, whereas IgE inhibition was not IL-12 independent and iii) established anti-ova IgE responses could be suppressed with BA+O/A (J. Immunol. 159:107-116, 1997). In a follow-up study we investigated what alternative pathway may account for the BA effect on down-regulating IgE. Th2 responses, such as those to 0/A are dependent on co-stimulation of CD28 on T cells via B7.2 on antigen-presenting-cells (APC). In contrast, Th1 stimulation appears to be dependent on interaction between B7.I on APC and CD28 on T cells. In order to determine whether BA could alter expression of co-stimulatory molecules and thereby down-regulate Th2 responses, we studied the effect of BA on co-stimulatory molecules in this system. We found that BA up-regulates B7.2 molecules on B cells (maximum at 24 h), but has no effect on B7.I. Interestingly, we demonstrated that BA increases expression of B7.2 on T cells (maximal at 42 h) and down- regulates expression of CD28 on T cells (maximal at 40 h). These findings may explain the observed effects of BA on the Th2 response, since down- regulation of CD28 may inhibit interaction between B7.2 on APC and CD28 on T cells. Unlike the Th2 response, the Th1 response was unaffected by the alteration in co-stimulatory molecules suggesting that Th1 stimulation is less dependent on this pathway, at least in the case of BA stimulation. In any event, these data provide an explanation for the ability of BA to inhibit IgE responses in an IL-12 independent fashion. They also suggest an approach that can be used to avoid allergic reactions to plasma derivatives by blocking interactions via CD28 on T cells. Pro-inflammatory cytokines, such as TNF-a, IL1-b, and IL-6 have been implicated as a cause of hypersensitivity symptoms. Bacterial cell wall constituents, such as lipopolysaccharide (LPS) and lipoteichoic acid are known to stimulate pro-inflammatory cytokine release in vitro from human monocytes. Recent studies have also shown that bacterial DNA (bDNA) and oligonucleotides containing specific bDNA sequences also detect bDNA in IGIV using a sensitive PCR method with primers specific for bacterial rRNA-encoding DNA sequences. Dose response experiments suggest that the amount of bDNA contained in IGIV is not sufficient to cause release of inflammatory cytokines. We plan to test DNA content of other CBER products to determine whether the levels could stimulate pro-inflammatory cytokines.